In knitting machines, yarn supply apparatuses have the task of supplying the appropriate knitting stations with yarn at the correct time, at the requisite tension, and in the desired amount. This is especially true for elastomeric yarns or other kinds of elastic yarns, which are predominantly processed in combination with hard or in other words essentially inelastic yarns (basic yarns) to make more or less elastic knitted goods. The tension of the elastomeric yarn substantially determines the feel and dimensional rigidity of the resultant knitted goods. Fluctuations in the tension of the elastomeric yarn supplied, especially when they recur systematically from one row of loops to another, can lead to a substantial impairment in quality of the knitted goods produced.
Because of the high expansion of often-used elastomeric yarns, which is up to 600% of the basic length, keeping the yarn tension constant requires an appropriate yarn supply apparatus, which furnishes the correct yarn quantity at a given time regardless of the yarn consumption at the time and regardless of the initial tension of the yarn paid out from a yarn bobbin.
This is true especially for knitting machines with an abruptly changing and at least sometimes very high yarn consumption, such as flatbed knitting machines or other knitting machines, in which a single yarn supply apparatus by itself supplies one row of needles. In flatbed knitting machines, the loop-forming needles arranged in one or more rows are supplied with one or more yarns to be knitted by means of a yarn guide moving back and forth in translational motion along the row of needles. Yarn supply is effected by means of a yarn supply apparatus which is located laterally next to the yarn guide in such a way that the yarn guide in its operating motion moves toward and away from the yarn supply apparatus. It will be appreciated that the requisite yarn supply quantity varies considerably in the two phases of operation. A further factor is that at the turning points between the two operating phases, zero yarn consumption occurs, and at the transition from the operating phase moving away from the yarn supply apparatus to the operating phase moving toward it, a brief interval of operation occurs in which the yarn travels backward.
For applications with yarn consumption that fluctuates greatly over time, the yarn supply apparatus known from German Patent DE 36 27 731 C1 was developed; it has a yarn wheel driven by a stepping motor. The yarn wheel carries the yarn, drawn from a yarn bobbin, to the applicable knitting station via a yarn brake.
The yarn supplied by the yarn wheel travels through a terminal eyelet of a lever supported pivotably on its other end; the eyelet represents a turning point, at which the yarn is rerouted at an acute angle. To adjust a constant yarn tension, the pivot lever is acted upon by a constant torque by means of a direct current motor. The pivot lever is also connected to a position transducer, which detects its pivoted position and readjusts the stepping motor accordingly. The pivot lever thus acts as a yarn store, for temporary storage of yarn that has not been drawn off by the knitting stations, yet has continued to be supplied because of the moment of inertia and the control characteristics of the stepping motor. It also serves to adjust the yarn tension and, in cooperation with the sensor device, to detect the existing yarn supply.
This yarn supply apparatus is only limitedly suitable for supplying elastic yarns, and the pivot lever proves to be overly insensitive for tension monitoring. Because of the intrinsic elasticity of the yarn, the pivot lever during operation reaches its extreme positions (stops), where the yarn tension is then not under control.
As a further development, the yarn supply apparatus for kinky and other effect yarns, known from German Patent DE 38 20 618 C2 is known; it has two rotationally driven yarn wheels, rotating in opposite directions, around which the yarn to be supplied is wrapped multiple times in a figure eight. An arm carrying an eyelet on its end and acted upon by torque in a predetermined direction of rotation acts as a yarn store for temporarily storing yarn intermittently not drawn off by the knitting stations. The yarn travels at an acute angle through its terminal eyelet, and for temporary storage it is deposited on bolts or posts located along a circle around the arm.
Frictional effects that affect yarn travel occur both on the bolts or posts forming a temporary store and at the eyelet of the arm through which the yarn travels at an acute angle.
From German Patent Disclosure DE 42 06 607 A1, a yarn supply apparatus for simultaneously supplying two yarns to a knitting machine is known, in which a yarn supply wheel is driven by a disk rotor motor. At least one yarn travels from the yarn supply wheel through the longitudinal opening of a helical spring wound in a conical or trumpet shape. A permanent magnet and a Hall sensor are provided on a bearing that pivotably holds the helical spring on one end, to enable detecting deflections of the helical spring. On the basis of these deflections, the disk rotor motor is readjusted, so that the command length of the helical spring is established in steady-state operation. In that position, the yarn travels laterally along the inner wall of the helical spring, through the opening in it. The helical spring acts as a spring and damping element, which allows a certain temporary storage of supplied yarn.
Finally, U.S. Pat. No. 3,858,416 discloses a yarn supply apparatus which is suitable for knitting machines that have substantially constant yarn consumption and for supplying hard yarns. The yarn supply apparatus has an electric motor whose rpm is controllable via the applied voltage and which by means of a suitable yarn wheel draws yarn from a bobbin and delivers it to the appropriate knitting station via a yarn tension sensor. A command value transducer is also present, which is connected to a command value input of a closed-loop controller, via a reversing switch and via selectively actuatable adjusting devices. Via the reversing switch, the controller receives a signal, characterizing the yarn tension, at its actual value input, and it readjusts the motor accordingly. Rpm sensors are also present on the electric motor and on the knitting machine; given a suitably different switch position of the reversing switch, they can be connected to the command value and actual value inputs of the controller. The reversing switch allows a switchover from one operating mode, with a yarn tension regulated so that it is constant, to an operating mode with a defined yarn supply quantity. Each knitting station of the circular knitting machine is assigned a corresponding yarn supply apparatus; so that the quantity of yarn to be supplied corresponds to the yarn consumption of a knitting station. The yarn travel speed is correspondingly low.
There are no provisions made for temporarily storing any possible excess lengths of yarn supplied as a result of motor inertia or motor characteristics or suddenly required to be paid out.
Such a yarn supply apparatus is not suitable for supplying elastic yarns to knitting machines that have a high yarn travel speed and abrupt changes in speed, of the kind that occur in flat bed knitting machines.